1. Field of the Invention
This invention relates to a central office protector module and more particularly to such a module which provides a visual indication when either of the lines protected by the module are shorted to ground.
2. Description of the Prior Art
Telephone operating companies use modules at their central offices to protect the equipment connected to each incoming line. The modules combine protection against excessive voltages with protection against sneak currents. The sneak currents are produced by voltages of relatively low magnitude as compared to the excessive voltages and usually result from accidental interference between telephone lines and adjacent power lines. While sneak currents are not strong enough to do any damage if they flow briefly they may generate enough heat to char insulation and do other damage if they persist.
Protection against excessive voltage is typically provided by a spark-gap protector which generally includes a pair of spaced carbon electrodes or a gaseous discharge device. The spark-gap protector is designed to provide protection against several occurrence of excessive voltage. Generally a solder pellet is included in series with the spark-gap protector so that the heat from a sustained overvoltage will cause the pellet to melt.
Protection against sneak current is typically provided by a device referred to as a heat coil. The heat coil includes a coil of small gauge, high resistance wire which is wound on a metal sleeve sometimes referred to as a bobbin. Inside the sleeve there is a contact pin which holds the bobbin in a predetermined position by a fusible bonding material such as solder. Should excessive currents occur on the line and persist, sufficient heat will be generated by the coil of wire to melt the solder and release the bobbin.
In many of the modules in use today the heat coil, spark-gap protector and solder pellet are typically arranged in series in a conductive enclosure also known as a cup. One end of the heat coil is accessible through the open end of the cup. The bobbin protrudes through that open end. The cup's contents are placed in contact with a line terminal or pin which is connected to the tip or ring conductor of the two wire telephone lines entering the central office. Thus the module provides one protection circuit for the tip conductor and an identical protection circuit for the ring conductor.
To complete the assembly, the module includes a ground terminal and a spring. The spring which is typically of the volute type is interposed between the closed end of the cup and the ground terminal. In the absence of a sustained overvoltage or persistent sneak current condition the cup does not come in contact with the line terminal.
Should a sustained overvoltage condition occur the heat from the operation of the spark-gap protector will cause the solder pellet to melt. The spring then urges the cup downward into contact with the line terminal to thereby ground the same. Should a persistent overcurrent condition occur the heat from the current flowing through the coil will cause the solder holding the bobbin in position to melt. The bobbin can no longer resist the downward force of the spring and the cup is then brought into contact with the line pin to thereby ground the same.
One example of a central office protector module of the type described above is that disclosed in U.S. Pat. No. 4,168,515 entitled "Line Protector For A Communications Circuit" which issued on Sept. 18, 1979 to B. Baumbach. The '515 patent is assigned to the same assignee as is the present invention.
The module of the '515 patent may be modified to have access holes in the module housing so that probes may be inserted into the housing for contact with the tip and ring circuits therein. Such a modification is necessary where the protector panel in which the module is mounted does not include a test field. In addition to the access holes the modified module includes two test terminals each associated with a respective one of the two line circuits. The test terminals are metallic and extend longitudinally in the module housing. One end of each of the test terminals is in electrical contact with the line terminals. The other end of each of the test terminals is in close proximity to the associated access hole. One example of the '515 module modified in the above described manner is disclosed in U.S. Pat. No. 4,502,088 entitled "Line Protector For A Communications Circuit" which issued on Feb. 26, 1985 to B. Baumbach and is assigned to the same assignee as is the present invention.
It is desirable to know at the central office when a persistent sneak current or sustained overvoltage has caused a module to ground either of the line pins. In order for that knowledge to be available to telephone company craftpersons the module must include a means to indicate the occurrence of a ground condition.
There have been several attempts in the prior art to provide such an indicating means in a central office module. Two such are disclosed in U.S. Pat. No. 3,587,021 entitled "Line Protector For A Communications Circuit" which issued on Jun. 22, 1971 and U.S. Pat. No. 3,794,947 entitled "Line Protector For A Communication Circuit" which issued on Feb. 26, 1974. Both the '021 and '947 patents are assigned to the same assignee as is the present invention.
The module of the '021 patent has an indicator which becomes visible when either of the two lines protected by the module sustains an overcurrent condition. For each line the module has an associated heat coil on a printed circuit board. The overcurrent condition allows an associated spring to push the circuit board outward to thereby ground the line. An extension of the circuit board protrudes from an opening in the neck of the housing to thereby indicate the overcurrent condition. The module also includes an alarm pin which is connected to ground when an overcurrent condition occurs. The grounding of the pin can be used to close a circuit in the central office such that an external audible or electric lamp alarm device can be energized.
The module of the '947 patent is an improvement of the module of the '021 patent. The '947 patent discloses two different forms for the module, one of which (shown in FIGS. 1-4) does not use the alarm pin or provide a visual indication and the other of which (shown in FIGS. 8-12) does use the alarm pin and provide a visual indication in the same manner as the module of the '021 patent. Such an alarm indicating module is sold by assignee's Reliable Electric/Utility Products operating unit as the R1104B module. That module plugs into a type 700 connector also sold by that operating unit.
The indicating means of the modules disclosed in the '947 and '021 patents cannot be easily used in the type of module disclosed in either the '515 or '088 patents. In addition, the prior art indicating means is quite complicated and costly to implement. Also the modules disclosed in the '947 and '021 patents have resettable heat coils whereas the module of either the '515 or '088 patents does not.
In summary it is not only desirable that a module of the type disclosed in either the '515 or '088 patents provide a visual indication that a persistent sneak current or sustained overvoltage has caused either line pin to be grounded, but that such indicating means be relatively simple in structure and easily implementable in a cost effective manner. The central office protector module of the present invention meets these requirements.